Stallings, the True South Classic winner in July for his second PGA Tour title, made eight birdie putts inside 10 feet in the unofficial event that features PGA, LPGA, Champions and Web.com tour players.
"I had to grind it all day, but didn't have to do anything too severe," said Stallings, who made an 8-foot par putt on the 18th. "Last year, I didn't know where to hit the ball. I didn't know the course. This time around, I felt more relaxed."
Stallings opened with a 76 last year, also at Del Monte, and ended up missing the cut.
McGirt, who had three top-10 finishes on the PGA Tour this season including tie for second in the Canadian Open, had a bogey-free 66 at Del Monte.
"I hit it pretty good, it was a solid day," McGirt said. "I had no bogeys and anytime who have no bogeys, it's a good day."
Triplett had a 66 Pebble Beach to top the 11 Champions Tour entrants. He won the 50-and-over tour's First Tee Open at Pebble Beach in July.
John Malinger, the 2010 Pebble Beach Invitational winner who also had three top-10 finishes on the PGA Tour this season, had a 67, also at Del Monte.
Champions Tour veteran Tom Purtzer was among five players at 68, with six others tied at 69, including 2011 U.S. Senior Open winner Olin Browne.
Annika Sorenstam, the Hall of Famer who retired from the LPGA Tour in 2008 and is playing in the event for the second in three years, shot a 70 at Del Monte -- the lowest score among the seven female entrants.
Defending tournament champion Harrison Frazar withdrew from event because of an undisclosed injury.
The final round will be played at Pebble Beach. The winner will receive $60,000 from the $300,000 purse.